As semiconductor device manufacturers continue to produce smaller devices, the requirements for photomasks used in the fabrication of these devices continue to tighten. Photomasks, also known as reticles or masks, typically consist of substrates that have an absorber layer formed on the substrate. The absorber layer includes a pattern representing a circuit image that may be transferred onto semiconductor wafers in a lithography system. As feature sizes of semiconductor devices decrease, the corresponding circuit images on the photomask also become smaller and more complex. Consequently, the quality of the mask has become one of the most crucial elements in establishing a robust and reliable semiconductor fabrication process.
In order to maintain the quality of the photomask throughout its lifetime, manufacturers have developed a pellicle to protect at least the patterned side of the photomask from being damaged by contaminants that may be present in semiconductor manufacturing tools. The pellicle typically includes a thin film attached to a frame, which has a height that places the thin film outside of the focal plane such that contaminants on the film are not imaged onto a semiconductor wafer.
Photomask contamination, however, may still occur under the pellicle film during a photomask and/or semiconductor manufacturing process. Organic materials or chemicals used in during the manufacturing processes may interact with photons to create airborne molecular contaminants (AMCs). Additionally, AMCs may be created from an outgas of materials during a lithography process. Other sources of AMCs may include vaporized photoresist that is released during a photoresist coating process, evaporation of different chemicals during pre-bake and post-bake processes, and solvents used on the photomask in a developing process.
Contaminants, such as AMCs, may enter into the space between the pellicle film and the photomask through, for example, the vent hole in the pellicle frame. Additionally, AMCs, or other contaminants, may be trapped under the pellicle film after the pellicle is mounted on the photomask. The AMCs and other contaminants may create a haze in the optical path associated with the photomask assembly, which can reduce the clarity of the image in the focal plane. For example, a layer of contaminants may build up on the patterned side of a photomask, which may darken and distort an image projected onto a wafer.
The semiconductor industry currently implements many techniques to protect photomask assemblies and photomask manufacturing tools from fine particles that have sizes in the microns. AMCs, however, may have sizes in the Angstroms and the conventional techniques may not be effective for eliminating contamination. For example, a conventional particulate filter may have a pore diameter of approximately 0.3 microns that allows AMCs, which may be as small as ten Angstroms, to pass through the filter into the photomask assembly or lithography tool.
One conventional technique used to remove particulates includes a chemical filter placed in a pellicle frame. The material forming the chemical filter may bind with the particulate matter in a chemical reaction to form a new composition. The new composition, however, may become another source of contamination. Additionally, the chemical filter may reach a saturation limit and stop reacting with the particulate matter, at which point the chemical filter no longer prevents the particulate matter from contaminating the lithography tools.
Vent holes formed in the pellicle frame may also be an access point for particulate matter and AMCs to enter the area between the pellicle film and the photomask. Conventional techniques of preventing particles from passing through the vent hole include adding filter sheets in the vent hold to block contaminants from entering the space under the pellicle film. Typically, these filter sheets have pore sizes that block particulates in the range of approximately 0.5 μm or greater and molecular contaminants in the range of approximately 0.02 μm or greater. Thus, molecular contaminants that are smaller than 0.02 μm may pass through the conventional filter.
Additionally, the filter sheets may only block contaminants entering through the vent hole. A fabrication and/or cleaning process may leave chemical residue on the photomask or pellicle. The chemical residue may react with other chemicals during a manufacturing process. As a result, molecular contaminants may form between the pellicle film and the photomask without being exposed to the filter sheets in the vent hole, which can form a haze in the optical path associated with the photomask assembly.